Mobile communication system

ABSTRACT

An object is to appropriately perform transmission power control on a mobile station (UE) when performing “Inter-eNB CA” by using cells under the control of plural radio base stations (eNB). In a mobile communication system according to the present invention, each of plural radio base stations (eNB#1, eNB#2) reduces, upon detecting that the mobile station (UE) is performing transmission at a maximum transmission power (P CMAX,c ), by a predetermined amount a transmission power of the mobile station (UE) in a cell (#1/#2A to #2C) under the control thereof and notifies other radio base station (eNB) of the predetermined amount.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.15/029,663 filed Apr. 15, 2016, which is a national stage application ofPCT/JP2014/077138, which claims priority to Japanese Patent ApplicationNo. 2013-215486. The contents of these applications are herebyincorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a mobile communication system.

BACKGROUND ART

In the LTE (Long Term Evolution) system, a mobile station UE#idetermines, based on a formula shown in FIG. 5A, a transmission powerP_(PUSCH,c)(i) of PUSCH (Physical Uplink Shared Channel) in a cell (orCC: Component Carrier) #c for every TTI (Transmission Time Interval).

Among the variables included in the formula shown in FIG. 5A,P_(CMAX,c)(i) is a maximum transmission power of the mobile station UE#iafter considering a required power back-off in a cell #c; M_(PUSCH,c)(i)is number of resource blocks assigned to the mobile station UE#i in thecell #c; P_(o PUSCH,c)(j) is a standard electric-power offset valuenotified in the cell #c; α_(c)(j) is an inclination value of FractionalTPC (Transmission Power Control) notified in the cell #c; PL_(c) is apropagation loss (path loss) in the cell #c measured by the mobilestation UE#i; Δ_(TF,c)(i) is an electric-power offset value determinedbased on a modulation method and a coding rate used in the mobilestation UE#i; and f_(c)(i) is a correction value for closed loop powercontrol used in the mobile station UE#i.

Among the variables included in the formula shown in FIG. 5A, thepropagation loss PL_(c) is unknown to a radio base station eNB.Therefore, when the propagation loss PL_(c) changes, the mobile stationUE#i feeds back PHR (Power Headroom Report) as shown in FIG. 5B.

PH is a difference between the maximum transmission power P_(CMAX,c)(i)of the mobile station UE#i and a current transmission powerP_(PUSCH,c)(i) of the mobile station UE#i calculated by using theformula shown in FIG. 5A. The calculated difference may be a positive ora negative value.

Moreover, in a mobile communication system of the LTE system, as shownin FIGS. 6A and 6B, an overlaying configuration in which small cells aredeployed on the coverage of a macrocell that uses an existing frequency(for example, 0.8/2 GHz) can be used.

The small cells, as shown in FIG. 6A, can operate at the same frequencyas that of the macrocell, or can operate at a different frequency (forexample, 3.5 GHz) from that of the macrocell, as shown in FIG. 6B.

Moreover, the small cells can be operated by a remote radio equipment(RRE) that is centrally controlled by a radio base station MeNB thatcontrols the macrocell, or can be operated by a radio base station SeNBthat differs from the radio base station MeNB, as shown in FIGS. 6A and6B.

The radio base station MeNB and the radio base station SeNB areconnected via a backhaul line, in which delay time cannot be ignored,and these radio base stations mutually exchange the information. Atransmission delay of the backhaul can be up to few tens ofmicroseconds.

Moreover, in Release-12 of the LTE system, CA (Carrier Aggregation) usedtill Release-10 of the LTE system has been expanded, and performing“Inter-eNB CA”, in which high throughput can be achieved by performing asimultaneous communication using cells (or CC) under the control ofplural radio base stations eNB that are connected via the backhaul isbeing studied (see Non-Patent Document 1).

PRIOR ART DOCUMENT Non-Patent Document Non-Patent Document 1: 3GPPContribution R2-131782 SUMMARY OF THE INVENTION

However, in the “Inter-eNB CA” that is currently being studied, amongplural radio base stations eNB, scheduling control or transmission powercontrol is independently performed, and therefore, had a problem whereeach radio base station eNB could not completely comprehend a status ofa transmission power of a mobile station UE#i.

Specifically, as shown in FIG. 7A, for each radio base station eNB,variables used in cells under the control of other radio base stationseNB, particularly, number of resource blocks M_(PUSCH,c)(i), apropagation loss PL_(c), an electric-power offset value Δ_(TF,c)(i), anda closed loop power control value f_(c)(i) are unknown. Therefore, asshown in FIG. 7B, there was a problem that each radio base station eNBcould not estimate, even after receiving the PHR in cells under thecontrol of other radio base stations eNB from a mobile station UE, thestatus of the transmission power of the mobile station UE#i in thecells.

The present invention is made in view of the above circumstances. Oneobject of the present invention is to provide a mobile communicationsystem that can appropriately perform transmission power control on amobile station UE when performing “Inter-eNB CA” by using cells underthe control of plural radio base stations eNB.

According to a first aspect of the present embodiment, in a mobilecommunication system in which a mobile station is capable of performingcarrier aggregation by using cells under the control of plural radiobase stations, each of the plural radio base stations reduces, upondetecting that the mobile station is performing transmission at amaximum transmission power, by a predetermined amount a transmissionpower of the mobile station in a cell under the control thereof andnotifies other radio base station of the predetermined amount.

According to a second aspect of the present embodiment, in a mobilecommunication system in which a mobile station is capable of performingcarrier aggregation by using cells under the control of plural radiobase stations, each of the plural radio base stations requests, upondetecting that the mobile station is performing transmission at amaximum transmission power, other radio base station to reduce atransmission power of the mobile station in a cell under the control ofthe other radio base station.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an overall structural diagram of a mobile communication systemaccording to a first embodiment of the present invention.

FIG. 2 is a functional block diagram of a radio base station eNB#1 oreNB#2 according to the first embodiment of the present invention.

FIG. 3 is a sequence diagram that shows an operation of the mobilecommunication system according to the first embodiment of the presentinvention.

FIG. 4 is a sequence diagram that shows an operation of a mobilecommunication system according to a first modification example of thepresent invention.

FIG. 5A demonstrates an expression for transmission power, and FIG. 5Bdemonstrates an expression for power head room.

FIG. 6A demonstrates a mobile communication system of the LTE systemwhere small cells may operate at the same frequency as that of themacrocell. FIG. 6B demonstrates a mobile communication system of the LTEsystem where small cells may operate at a different frequency as that ofthe macrocell is a view for explaining a conventional technology.

FIG. 7A demonstrates an expression for transmission power, and FIG. 7Bdemonstrates an expression for power head room.

DETAILED DESCRIPTION

Mobile Communication System According to First Embodiment of PresentInvention

Referring now to FIGS. 1 to 3, a mobile communication system accordingto a first embodiment of the present invention will be explained below.The mobile communication system according to the present embodiment willbe explained with an example of a mobile communication system of the LTEsystem. The present invention, however, can be applied to a mobilecommunication system other than that of the LTE system.

As shown in FIG. 1, the mobile communication system according to thepresent embodiment includes a radio base station eNB#1 that controls acell #1, and a radio base station eNB#2 that controls cells #2A to #2C.

In the mobile communication system according to the present embodiment,the radio base station eNB#1 is a master radio base station (or, a macroradio base station) MeNB, and the radio base station eNB#2 is a slaveradio base station (or, a small radio base station) SeNB.

Moreover, in the mobile communication system according to the presentembodiment, the cell #1 is a macrocell, and the cells #2A to #2C aresmall cells. In the cell #1, CC#1 is used, and in the cells #2A to #2C,CC#2 is used.

Furthermore, in the mobile communication system according to the presentembodiment, a mobile station UE is capable of performing “Inter-eNB CA”by using the cell #1 (CC#1) under the control of the radio base stationeNB#1 and the cells #2A to #2C (CC#2) under the control of the radiobase station eNB#2.

As shown in FIG. 2, the radio base station eNB#1 or eNB#2 includes acontrolling unit 11, a receiving unit 12, and a transmitting unit 13.

The controlling unit 11 of the radio base station eNB#1 is capable ofperforming control on communication with the mobile station UE or theradio base station eNB#2. The receiving unit 12 of the radio basestation eNB#1 is capable of receiving various signals from the mobilestation UE or the radio base station eNB#2. The transmitting unit 13 ofthe radio base station eNB#1 is capable of transmitting various signalsto the mobile station UE or the radio base station eNB#2.

The controlling unit 11 of the radio base station eNB#2 is capable ofperforming control on communication with the mobile station UE or theradio base station eNB#1. The receiving unit 12 of the radio basestation eNB#2 is capable of receiving various signals from the mobilestation UE or the radio base station eNB#1. The transmitting unit 13 ofthe radio base station eNB#2 is capable of transmitting various signalsto the mobile station UE or the radio base station eNB#1.

In the radio base station eNB#1, the controlling unit 11 is capable ofreducing, upon detecting that the mobile station UE is performingtransmission at a maximum transmission power P_(CMAX,c) (so-called powerceiling level), by a predetermined amount a transmission power of themobile station UE in the cell #1 under the control thereof, and thetransmitting unit 13 is capable of notifying the radio base stationeNB#2 of the predetermined amount.

Similarly, in the radio base station eNB#2, the controlling unit 11 iscapable of reducing, upon detecting that the mobile station UE isperforming transmission at a maximum transmission power P_(CMAX,c), by apredetermined amount a transmission power of the mobile station UE inthe cells #2A to #2C under the control thereof, and the transmittingunit 13 is capable of notifying the radio base station eNB#1 of thepredetermined amount.

The controlling unit 11 in the radio base station eNB#1 or eNB#2 iscapable of detecting, based on the PHR transmitted by the mobile stationUE, that the mobile station UE is performing transmission at the maximumtransmission power P_(CMAX,c).

For example, the controlling unit 11 in the radio base station eNB#1 oreNB#2 can reduce, by reducing the number of resource blocks to beassigned to the mobile station UE, by a predetermined amount thetransmission power of the mobile station UE.

Moreover, the controlling unit 11 in the radio base station eNB#1 oreNB#2 is capable of further reducing, upon still detecting that themobile station UE is performing transmission at the maximum transmissionpower P_(CMAX,c) even after reducing by the predetermined amount thetransmission power of the mobile station UE, by a predetermined amountthe transmission power of the mobile station UE.

Furthermore, determination regarding whether the mobile station UE isperforming transmission at the maximum transmission power P_(CMAX,c) ornot (in other words, determination of power ceiling level for theabove-mentioned transmission power of the mobile station UE) can beperiodically performed.

Moreover, the reduction amount of the transmission power of the mobilestation UE can be changed step-by-step. According to the abovecharacteristic, by increasing the reduction amount of the transmissionpower of the mobile station UE step-by-step, the attainment of the powerceiling level for the transmission power of the mobile station UE can beprevented at an early stage.

The transmitting unit 13 in the radio base station eNB#1 can notify theradio base station eNB#2 of the above-mentioned predetermined amount viaan X2 interface, or can notify the radio base station eNB#2 of theabove-mentioned predetermined amount via the mobile station UE.

Referring now to FIG. 3, an example of how the mobile communicationsystem according to the present embodiment operates will be explainedbelow.

As shown in FIG. 3, at step S1001, the mobile station UE transmits thePHR to the radio base station eNB#1.

At step S1002, based on the received PHR, the radio base station eNB#1reduces, upon detecting that the mobile station UE is performingtransmission at a maximum transmission power P_(CMAX,c), by apredetermined amount a transmission power of the mobile station UE inthe cell #1 under the control thereof. At step S1003, the radio basestation eNB#1 notifies the radio base station eNB#2 of the predeterminedamount.

First Modification Example

Referring now to FIG. 4, a mobile communication system according to afirst modification example of the present invention will be explainedbelow while focusing on the points of difference with the mobilecommunication system of the first embodiment.

In the mobile communication system according to the first modificationexample, when the controlling unit 11 in the radio base station eNB#1detects that the mobile station UE is performing transmission at amaximum transmission power P_(CMAX,c) the transmitting unit 13 requeststhe radio base station eNB#2 to reduce the transmission power of themobile station UE in the cells #2A to #2C under the control of the radiobase station eNB#2.

Similarly, when the controlling unit 11 in the radio base station eNB#2detects that the mobile station UE is performing transmission at amaximum transmission power P_(CMAX,c), the transmitting unit 13 requeststhe radio base station eNB#1 to reduce the transmission power of themobile station UE in the cell #1 under the control of the radio basestation eNB#1.

The transmitting unit 13 can notify, while performing the request, of anamount of the transmission power of the mobile station UE desired to bereduced.

Moreover, when the controlling unit 11 in the radio base station eNB#1reduces, as per the above-mentioned request, the transmission power ofthe mobile station UE in the cell #1, the transmitting unit 13 canreturn ACK to the radio base station eNB#2.

On the other hand, if the controlling unit 11 in the radio base stationeNB#1 cannot reduce, as per the above-mentioned request, thetransmission power of the mobile station UE in the cell #1, thetransmitting unit 13 can return NACK to the radio base station eNB#2, orcan notify the radio base station eNB#2 of an amount of the transmissionpower that can be reduced.

Similarly, when the controlling unit 11 in the radio base station eNB#2reduces, as per the above-mentioned request, the transmission power ofthe mobile station UE in the cells #2A to #2C, the transmitting unit 13can return an ACK to the radio base station eNB#1.

On the other hand, if the controlling unit 11 in the radio base stationeNB#2 cannot reduce, as per the above-mentioned request, thetransmission power of the mobile station UE in the cells #2A to #2C, thetransmitting unit 13 can return NACK to the radio base station eNB#1, orcan notify the radio base station eNB#1 of a maximum amount of thetransmission power that can be reduced.

The transmitting unit 13 in the radio base station eNB#1 can return theabove-mentioned request, ACK, NACK, or notification to the radio basestation eNB#2 via the X2 interface, or can return the above-mentionedrequest, ACK, NACK, or notification to the radio base station eNB#2 viathe mobile station UE.

Similarly, the transmitting unit 13 in the radio base station eNB#2 canreturn the above-mentioned request, ACK, NACK, or notification to theradio base station eNB#1 via the X2 interface, or can return theabove-mentioned request, ACK, NACK, or notification to the radio basestation eNB#1 via the mobile station UE.

Referring now to FIG. 4, an example of how the mobile communicationsystem according to the first modification example operates will beexplained below.

As shown in FIG. 4, at step S2001, the mobile station UE transmits thePHR to the radio base station eNB#1.

At step S2002, the radio base station eNB#1 detects, based on thereceived PHR, that the mobile station is performing transmission at themaximum transmission power P_(CMAX,c). At step S2003, the radio basestation eNB#1 requests the radio base station eNB#2 to reduce thetransmission power of the mobile station UE in the cells #2A to #2C.

At step S2004, the radio base station eNB#2 reduces the transmissionpower of the mobile station UE in the cells #2A to #2C. At step S2005,the radio base station eNB#2 returns ACK to the radio base stationeNB#1.

The characteristic of the present embodiments explained above can beexpressed as follows.

According to a first aspect of the present embodiment, in a mobilecommunication system in which a mobile station UE is capable ofperforming “Inter-eNB CA (carrier aggregation)” by using cells #1 and#2A to #2C under the control of plural radio base stations eNB#1 andeNB#2, each of the plural radio base stations eNB#1 and eNB#2 reduces,upon detecting that the mobile station UE is performing transmission ata maximum transmission power P_(CMAX,c), by a predetermined amount atransmission power of the mobile station UE in a cell #1 or #2A to #2Cunder the control thereof and notifies other radio base station eNB ofthe predetermined amount.

According to the above aspect, when the mobile station UE is performingthe “Inter-eNB CA” by using the cells #1 and #2A to #2C under thecontrol of the radio base stations eNB#1 and #2, the radio base stationeNB#1 or eNB#2 can appropriately perform transmission power control onthe mobile station UE without knowing a status of a transmission powerin other radio base station eNB corresponding to the mobile station UE.

In the above first aspect of the present embodiment, each of the pluralradio base stations eNB#1 and eNB#2 can perform the notification via themobile station UE.

According to the above aspect, because the notification is nottransmitted via a backhaul line between the radio base station eNB#1 andthe radio base station eNB#2, a propagation delay corresponding to thenotification can be evaded.

According to a second aspect of the present embodiment, in a mobilecommunication system in which a mobile station UE is capable ofperforming “Inter-eNB CA” by using cells #1 and #2A to #2C under thecontrol of plural radio base stations eNB#1 and eNB#2, each of theplural radio base stations eNB#1 and eNB#2 requests, upon detecting thatthe mobile station UE is performing transmission at a maximumtransmission power P_(CMAX,c), other radio base station eNB to reduce atransmission power of the mobile station UE in a cell under the controlof the other radio base station eNB.

According to the above aspect, when the mobile station UE is performingthe “Inter-eNB CA” by using the cells #1 and #2A to #2C under thecontrol of the radio base stations eNB#1 and #2, the radio base stationeNB#1 or eNB#2 can appropriately perform the transmission power controlon the mobile station UE without knowing a status of a transmissionpower in the other radio base station eNB corresponding to the mobilestation UE.

In the above second aspect of the present embodiment, the request caninclude an amount of the transmission power desired to be reduced of themobile station UE.

According to the above aspect, an excessive reduction of thetransmission power of the mobile station UE can be evaded in the radiobase station eNB#1 or eNB#2.

In the above second aspect of the present embodiment, each of the pluralradio base stations eNB#1 and eNB#2 can perform the request via themobile station UE.

According to the above aspect, because the notification is nottransmitted via a backhaul line between the radio base station eNB#1 andthe radio base station eNB#2, a propagation delay corresponding to therequest can be evaded.

The operations of the radio base station eNB#1, eNB#2, or the mobilestation UE can be realized by hardware such as a circuit, can berealized by a software module executed by a processor, or can berealized by the combination of these.

The software module can be arranged in a storage medium having a desiredform such as a RAM (Random Access Memory), a flash memory, a ROM (ReadOnly Memory), an EPROM (Erasable Programmable ROM), an EEPROM(Electronically Erasable and Programmable ROM), a register, a hard disk,a removable disk, a CD-ROM, and the like.

The storage medium is connected to a processor so that the processor canread/write information from/in the storage medium. Alternatively, thestorage medium can be integrated in a processor. Alternatively, thestorage medium and the processor can be arranged in ASIC. The ASIC canbe arranged in the radio base station eNB#1, eNB#2, or the mobilestation UE. The storage medium and the processor can be arranged as adiscrete component in the radio base station eNB#1, eNB#2, or the mobilestation UE.

The present invention has been explained in detail by using theabove-mentioned embodiments; however, it is obvious for a person skilledin the art that the present invention is not limited to the embodimentsexplained in the present description. The present invention can beimplemented byway of modifications and changes without deviating fromthe gist and the range of the present invention specified by the claims.Accordingly, the indication of the present description aims at exemplaryexplanation, and has no intention to limit to the present invention.

The entire contents of Japanese Patent Application 2013-215486 (filed onOct. 16, 2013) are incorporated in the description of the presentapplication by reference.

INDUSTRIAL APPLICABILITY

As explained above, according to the present invention, it is possibleto provide a mobile communication system that can appropriately performtransmission power control on a mobile station UE when performing“Inter-eNB CA” by using cells under the control of plural radio basestations eNB.

EXPLANATION OF REFERENCE NUMERALS

-   UE Mobile station-   eNB#1/eNB#2 Radio base station-   11 Controlling unit-   12 Receiving unit-   13 Transmitting unit

1. A mobile communication system in which a mobile station performscarrier aggregation by using cells under the control of plural radiobase stations, wherein at least one of the plural radio base stationsrequests another radio base station to change a transmission power ofthe mobile station in a cell under the control of the other radio basestation.
 2. The mobile communication system according to claim 1,wherein the request contains an amount of the transmission power desiredto be changed of the mobile station.
 3. The mobile communication systemaccording to claim 1, wherein the at least one of the plural radio basestations makes the request via the mobile station.